Process for the oxidation of alcohols to acids

ABSTRACT

The instant invention relates to a process for the oxidation of primary alcohols to the corresponding acids, which comprises contacting and thereby reacting an alcohol with a tertiary amine oxide compound at elevated temperatures in the presence of a homogeneous oxidation catalyst comprising a Group VIII metal and a promoter comprising a quaternary ammonium bromide, and subsequently separating out acids from the reaction mixture product.

FIELD OF THE INVENTION

This invention relates to an improved process for the oxidation ofalcohols to acids using a tertiary amine oxide oxygen activator in thepresence of a homogeneous oxidation catalyst and a promoter.

BACKGROUND OF THE INVENTION

Alcohols, particularly primary alcohols, are readily available in largecommercial quantities. Processes to convert primary alcohols to acidsfill a useful need in the industrial world. Acids have been usedcommercially for the manufacture of synthetic lubricants and soaps.

In the past, various processes have been developed for the production ofacids. It has been proposed to treat various alcohols with an oxidizingagent in the presence of a catalyst, such as ruthenium and a solvent toproduce the corresponding acids. Such processes, however, have generallyproduced high yields of aldehydes and rather low yields of acids. Sinceprocesses that produce high yields of acids are particularly desired, itis an object of this invention to provide a process for the conversionof alcohols to the corresponding carboxylic acids having a highselectivity to acids.

It has been found that high selectivity to acids can be achieved by aprocess for oxidizing alcohols to acids which comprises reacting analcohol with a tertiary amine oxide oxidant at elevated temperatures inthe presence of an oxidation catalyst comprising a Group VIII metal anda promoter comprising a quaternary ammonium bromide and thereafterseparating out the acids formed from the reaction product mixture.

SUMMARY OF THE INVENTION

The instant invention relates to an improved process for the oxidationof primary alcohols to acids, which comprises contacting and therebyreacting at elevated temperatures an alcohol with a tertiary amine oxidecompounds in the presence of an oxidation catalyst comprising a GroupVIII metal and a promoter comprising a quaternary ammonium bromide, andsubsequently separating out acids from the reaction mixture product.

DETAILED DESCRIPTION OF THE INVENTION

The instant process comprises reacting a primary alcohol with a tertiaryamine oxide oxidant at elevated temperatures in the presence of anoxidation catalyst comprising a Group VIII metal and a promotercomprising a quaternary ammonium bromide.

Alcohols typically used in the process of the present invention areprimary mono-hydric aliphatic alcohols. The aliphatic alcoholspreferably have from one to about 30 carbon atoms, with C₆ to C₂₄alcohols considered more preferred and C₈ to C₂₀ alcohols consideredmost preferred. As a general rule, the aliphatic alcohols are primarilyof straight-chain structure. Specific examples of primary straight-chainmonohydric aliphatic alcohols include ethanol, hexanol, octanol,dodecanol, pentadecanol and octadecanol.

Mixtures of alcohols are also suitable for purposes of this invention.Mixtures of higher (e.g., C₈ to C₂₀) monohydric aliphatic alcohols areknown to be commercially prepared, for instance, by hydroformylation ofolefins or by reduction of naturally occurring fatty esters. Specificexamples of commercially available alkanol mixtures in the C₉ to C₁₈range are the NEODOL detergent alcohols, trademark of and manufacturedby Shell Chemical Company. e.g., the products identified as NEODOL 91alcohols (predominantly in the C₉ to C₁₁ range), NEODOL 23 alcohols(predominantly in the C₁₂ to C₁₃ range), NEODOL 25 alcohols(predominantly in the C₁₂ to C₁₅ range) and NEODOL 45 alcohols(predominantly in the C₁₄ to C₁₅ range)

The oxidant in the process of the instant invention is a tertiary amineoxide compound. The tertiary amine oxide oxidant reacts with a primaryalcohol at temperatures in the range of from about 40° C. to about 200°C. and converts it to primarily acids, although some aldehydes andesters are also formed. The process of the instant invention can also becarried out in the presence of an additional oxidant such as oxygen orair, or in an inert atmosphere.

The tertiary amine oxides suitable for use in the process of the instantinvention are those having the general formula: ##STR1## wherein R₁ andR₂ are alkyl groups, preferably methyl groups or ethyl groups, which arethe same or different, and R₃ is a phenyl group or alkyl substitutedphenyl group, a benzyl group or an alkyl substituted benzyl group, acyclohexyl group or an alkyl substituted cyclohexyl group, acyclohexanemethyl group or an alkyl substituted cyclohexanemethyl group;or wherein R₁ is an alkyl group, preferably a methyl group or an ethylgroup, and N, R₂ and R₃ together form part of a saturated or unsaturatedheterocyclic group, or where N, R₁, R₂ and R₃ together form part of asaturated or unsaturated heterocyclic group or bridged heterocyclicgroup.

When N, R₂ and R₃, or N, R₁, R₂ and R₃ form a heterocyclic or bridgedheterocyclic group, a second tertiary amine oxide group can alsooptionally be included in the heterocyclic or bridged heterocyclicgroup.

Examples of suitable tertiary amine oxides include: N,N-dimethylbenzylamine oxide; N,N-diethyl benzylamine oxide; N-methyl N-ethylbenzylamine oxide; pyridine N-oxide; α-picoline N-oxide; β-picolineN-oxide; γ-picoline N-oxide; pyrazine N-oxide; pyrazine N,N'-dioxide;N-methyl piperidine N-oxide; N-ethyl piperidine N-oxide; N-methylpiperazine N-oxide; N,N'-dimethyl piperazine N,N'-dioxide;triethylenediamine N-oxide; triethylenediamine N,N'-dioxide;N-methylmorpholine N-oxide; N-ethyl morpholine N-oxide; quinolineN-oxide; N-methyl pyrrole N-oxide; N-methyl pyrrolidine N-oxide;N,N-dimethyl cyclohexylamine N-oxide; N, N-dimethylcylcohexanemethylamine N-oxide; and N,N-dimethylaniline N-oxide. In apreferred embodiment, the tertiary amine oxide oxidant isN-methylmorpholine N-oxide.

The tertiary amine oxide oxidant should be present in an amountsufficient to obtain complete oxidation of the alcohol to an acid.Typically, the tertiary amine oxide is used in an amount sufficient tosupply between about 0.4 and about 1.8 mole equivalents of oxygen permole of alcohol.

The tertiary amine oxide may be utilized in a one-phase hydrocarbonsystem or in a two-phase hydrocarbon/water system. The solubility of thetertiary amine oxide in the aqueous phase will depend on the presence ofwater solubilizing substituents on the tertiary amine oxide, such as,for example, alkylsulfonates, arylsulfonates and the like. The use of atwo phase system can have certain processing advantages, particularlywhen the tertiary amine oxide is more soluble in the aqueous phase thanin the hydrocarbon phase and the alcohol and the product acids,aldehydes and esters are more soluble in the hydrocarbon phase than inthe aqueous phase. In the latter case, the organic phase containsunreacted alcohol and the major portion of the product acids, aldehydesand esters and the aqueous phase contains the tertiary amine oxide and aresidual amount of product acids, aldehydes and esters, which will allowa ready separation of product acids, aldehydes and esters and reactantalcohols from the oxygen activator. Long chain alcohols are suitablyprocessed with this two phase system.

In addition to the use of water as a solvent as indicated above, otherorganic solvents can be utilized, such as alkanes, aromatics, such asbenzene, toluene and xylene; alkanes, halo-substituted aromatics,amides, amines, ethers, sulfoxides. ketones, etc.. Solvents selectedshould not react with either the reactant alcohol or the product acids.

The catalyst utilized in accordance with the present invention ishomogeneous oxidation catalyst comprising a Group VIII metal. With thecatalyst being homogeneous, it is preferred that a two phasehydrocarbon/water system be utilized and that the catalyst be soluble inthe aqueous phase in order to facilitate separation of the catalyst fromthe product acids, aldehydes and esters. Preferably, the Group VIIImetal is selected from the group consisting of ruthenium, rhodium,platinum, palladium and mixtures thereof. In a particularly preferredembodiment, the Group VIII metal is ruthenium. The amount of catalystutilized in the present process is generally in amounts the range offrom about 0.1 percent by weight to about 15 percent by weight, basissubstrate alcohol and preferably, from about 1 percent by weight toabout 8 percent by weight.

The process of the instant invention is also carried out in the presenceof a promoter. A promoter comprising a quaternary ammonium bromide isutilized. Preferably, the promoter is quaternary alkylammonium bromidewherein the alkyl moieties have carbon numbers ranging from 1 to about20. Higher alkylammonium bromides are preferred. Suitable quaternaryalkylammonium bromides include didecyldimethylammonium bromide,tetrapropylammonium bromide, cetyldimethylethylammonium bromide andcetyltrimethylammonium bromide, with didecyldimethylammonium bromidebeing preferred. The promoter is typically used in amounts rangingbetween about 1 percent by weight and about 100 percent by weight, basissubstrate alcohol, preferably, between about 10 percent by weight andabout 80 percent by weight, and more preferably, between about 50percent by weight and about 60 percent by weight.

The alcohol oxidation reaction may be carried out in a batch reactor orin a continuous flow reactor. For example, the alcohol and the tertiaryamine oxide oxidant may be trickled over a bed of inert supportmaterials, such as alumina raschig rings or berl saddles, in thepresence of air or nitrogen and in the presence of the homogeneouscatalyst. Other continuous reactor configurations will be readilyapparent to one skilled in the art.

Batch reactors, such as autoclaves, are also suitable. For example, theprimary alcohol, an aqueous solution of the tertiary amine oxide and thehomogeneous oxidation catalyst can be loaded into an autoclave, theautoclave sealed and charged with an oxygen-containing gas, heated to adesired reaction temperature and the reaction allowed to proceed.

The process in the instant invention is carried out at elevatedtemperatures. The reaction temperature is typically in the range of fromabout 40° C. to 200° C., preferably from about 50° C. to about 100° C.and more preferably, from about 55° C. to about 65° C. While thereaction will proceed at lower temperatures, temperatures greater thanabout 40° C, preferably greater than about 50° C., are necessary inorder to obtain a high selectivity to acids. Reaction pressures for theinstant process are not critical and will typically range from aboutatmospheric to about 150 atmospheres, preferably from about 35atmospheres to about 100 atmospheres although higher and lower pressurescan be utilized.

For the two-phase system, after reaction there will be a hydrocarbonphase containing unreacted alcohol and the major portion of the productacids, aldehydes and esters. The hydrocarbon phase may be diluted withan inert organic solvent, such as an alkane. There will also be anaqueous phase containing the oxidized tertiary amine oxide, promoter, asmall amount of alcohol and product acids, aldehydes and esters, and thehomogeneous oxidation catalyst. The hydrocarbon phase is processed, forexample, by distillation, to recover the product acids, aldehydes andesters. The product acids, aldehydes and esters may then be distilled torecover the product acids. The aqueous phase may also be processed toremove any residual alcohol or acids/aldehydes/esters, say byliquid-liquid extraction with an organic solvent.

For the single phase hydrocarbon system, after reaction, the hydrocarbonphase is processed by conventional techniques such as distillation,liquid-liquid extraction with water, filtration, etc; in order toseparate the product acids, aldehydes and esters, the oxidized tertiaryamine oxide, unreacted alcohol and any oxidation catalyst and promoter.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the instant invention. It is, however, understood thatother ranges and limitations that perform substantially the samefunction in substantially the same way to obtain the same orsubstantially the same result are intended to be within the scope of theinstant invention as defined by the instant specification and claims.

The invention will be described by the following examples which areprovided for illustrative purposes and are not to be construed aslimiting the invention.

EXPERIMENTAL PROCEDURE

In a typical experiment a round bottom flask (50 ml capacity), equippedwith a stir bar and a nitrogen (1 atmosphere) line capping the neck, wascharged in a dry box (pre-purified nitrogen-gas purged) with 10.0 gramsof solvent (for example, acetone); 1.56 grams (8.37 mmols) of alcoholsubstrate (for example, 1-dodecanol); 0.50 grams of dodecane; 0.98 grams(8.37 mmols) of tertiary amine oxide (for example, N-methylmorpholine-N-oxide); and 0.13 grams of RuCl₃ ×H₂ O catalyst. In theexperiments containing a promoter, 0.94 grams (2 mmols) of quaternaryammonium bromide (for example, didecyldimethylammonium bromide) wasadded.

The round bottom flask was then removed from the dry box andmagnetically stirred and charged with a continuous flow of 1 atmosphereof nitrogen. The flask was then heated to 65° C. (internal temperature)while maintaining rapid mechanical for a period of about 2 hours untilno further pressure drop occurred. The hot flask was then allowed tocool down to room temperature.

The solution of the crude reaction product mixture was filtered throughglass wool and then analyzed by gas liquid chromatography (GLC) using aHewlett-Packard 5890A gas chromatograph fitted with a hydrogen flameionization detector and a data processor. The peaks of the chromatogramswere identified qualitatively by comparison of their retention timeswith those of standards and by GC/MS technique. The quantitativeestimation of the percent composition of the reaction mixture wasobtained from the integration of the areas of the peaks in thechromatograms.

ILLUSTRATIVE EMBODIMENT I

Following the experimental procedure described above experiments werecarried out in the presence and the absence of a promoter. Theexperimental conditions and the results are shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Oxidation of Alcohol:Effect of Promoter                                       Pro-       Conver-  Selectivity, %                                            Substrate                                                                            moter   sion, %  C.sub.12 -Acid                                                                       .sub.12 -Aldehyde                                                                      C.sub.24 -Ester                       ______________________________________                                        1-Do-  DDAB    51.88    57.72   19.0    0.0                                   decanol                                                                       1-Do-  None    31.65    0.0    100.0    0.0                                   decanol                                                                       ______________________________________                                        Conditions                                                                    Solvent       Acetone, 10.0 grams                                             Promoter      DDAB is Didecyldimethylammonium                                               bromide, 0.94 g (2 mmols)                                       Catalyst      RuCl.sub.3 × H.sub.2 O, 0.13 g (0.63 mmols)               Oxidant       N-methylmorpholine-N-oxide (0.98 g,                                           8.37 mmols)                                                     Oxidation Temp.                                                                             65° C.                                                   Oxidation Reaction Time                                                                     2 hrs.                                                          ______________________________________                                    

What is claimed is:
 1. A process for the oxidation of primary alcoholsto the corresponding acids, which comprises contacting and therebyreacting an alcohol with a tertiary amine oxide compound at elevatedtemperatures in the presence of a homogeneous oxidation catalystcomprising a Group VIII metal and a promoter comprising a quaternaryammonium bromide, and subsequently separating out acids from thereaction mixture product.
 2. The process of claim 1 wherein theoxidation is carried out at a temperature ranging from about 40° C. toabout 200° C.
 3. The process of claim 2 wherein the oxidation is carriedout at a temperature ranging from about 50° C. to about 100° C.
 4. Theprocess of claim 1 wherein said alcohol is a primary mono-hydricaliphatic alcohols.
 5. The process of claim 4 wherein said alcoholcontains from one to about 30 carbon atoms.
 6. The process of claim 5wherein said alcohol contains from about 8 about 20 carbon atoms.
 7. Theprocess of claim 1 wherein said Group VIII metal is selected from thegroup consisting of ruthenium, rhodium, platinum, palladium and mixturesthereof.
 8. The process of claim 7 wherein said Group VIII metal isruthenium.
 9. The process of claim 8 wherein said catalyst is rutheniumchloride.
 10. The process of claim 1 wherein the promoter is aquaternary alkylammonium bromide.
 11. The process of claim 10 whereinthe quaternary alkyl ammonium bromide is didecyldimethylammoniumbromide.
 12. The process of claim wherein the tertiary amine oxidecompound is N-methylmorpholine N-oxide.
 13. A process for the oxidationof primary alcohols to the corresponding acids, which comprisescontacting and thereby reacting an alcohol with a tertiary amine oxidecompound at a temperature in the range of from about 50° C. to about100° C. in the presence of a homogeneous oxidation catalyst comprisingruthenium and a promoter comprising a quaternary alkylammonium bromide,and subsequently separating out acids from the reaction mixture product.14. The process of claim 13 wherein the quaternary alkylammonium bromideis didecyldimethylammonium bromide.
 15. The process of claim 13 whereinthe tertiary amine oxide compound is N-methylmorpholine N-oxide.